The present invention relates to electrical submersible pumps for use in pumping oil from the borehole of a well to the surface.
An example of a known electrical submersible pump assembly is disclosed in commonly owned U.S. Pat. No. 6,120,261. The assembly includes an electric motor section and a pump section. A hollow drive shaft has one end fixed to the rotor of the motor. The other end of the drive shaft extends into the pump portion of the assembly, to rotate pump impellers contained in a pump housing. Oil from the reservoir enters the hollow drive shaft, and is drawn into the lower end of the pump housing through a series of holes formed in the shaft. The impellers pump the oil upwardly to the upper end of the pump housing, where the oil is forced through another series of holes back into the hollow shaft. The upper end of the drive shaft is connected to piping for thereafter delivering the oil to the surface.
While submersible pumps of the type described in the '261 patent are widely used in the oil industry, such pumps may not be suitable for use in wells having a high gas-to-oil ratio, i.e., wells having a gas/oil ratio exceeding approximately 60% free gas. Even at 60%, pumping can be effected only by installing additional special pump stages to the pump where the fluids are mixed and prepared for the main pump stages. However, in some high gas-to-oil ratio wells, irregular flows of 100% gas may occur, and even the just-described measures may be ineffective.
Currently, high gas-to-oil wells with wellhead pressure can flow freely to the processing facility without boosting. When the pressure is not sufficient, the wells will cease to flow and production losses occur across the field.
Surface horizontal multi-phase pumps is a known solution but can have drawbacks. Such systems require upgrading surface facilities (pipe lines, new site, electric power, etc.) to install the large, rotating equipment needed.